1. Field of the Invention
The present invention relates to liquid-crystal display devices. More particularly, the present invention relates to a driving method using electro-optical response characteristic and bistability of a liquid crystal.
2. Description of the Related Art
Liquid-crystal display devices have features such as lower weight, thinner structures, and lower power consumption than display devices using CRTs, and are used as monitor displays for portable phones and portable information terminals. There have been strong demands for the displays in portable phones and portable information terminals to be lightweight and thin and to have low power consumption. In particular, in portable phones having a function for displaying information such as characters and symbols even when waiting for a call (at reception waiting time), since a lower power consumption display greatly contributes to the battery life, it is important to develop lower power consumption devices.
Liquid-crystal display devices are broadly classified into transmission types using a backlight and reflection types using reflection of external light. In a transmission-type liquid-crystal display device, since an backlight for illumination is always turned on, the power consumption is large, and therefore the transmission-type liquid-crystal display device is not suitable for display applications of portable devices. At present, reflection-type liquid-crystal display devices which do not require a backlight are mainly used. However, in a reflection-type liquid-crystal display device too, rewriting of the screen is performed periodically at a predetermined frame frequency in an active-matrix technique. For this reason, merely continuing to display a still image and characters consumes a fixed amount of power because the frame is updated, causing the battery lifetime to be shortened. In order to ensure the battery lifetime for future portable devices, which are expected to be increasingly sophisticated, a still more reduction in the power consumption of the display is strongly desired.
Until now, several technologies have been proposed for the problems such as those described above. For example, in Japanese Unexamined Patent Application Publication No. 2000-214466, D. C. Ulrich, et al., Proc. IDW' 00, PLC1–4 (2000) p.293, and J. C. Jones, et al., Proc. IDW' 00, PLC2-2 (2000) p.301, in a liquid-crystal display device with a simple matrix structure, a display mode using bistability of the liquid crystal has been proposed. In this display mode, bistability (memory characteristic) possessed by the liquid crystal itself is used, and the display is maintained even if the applied voltage is removed. Therefore, there is an advantage in that, when a still image is displayed, no power is consumed due to the memory characteristic of the liquid crystal itself. The liquid crystal maintains two states (bistable states) with different transmittances when no voltage is applied, and by using this phenomenon, a two-gradation display of black and white can be made. In addition, if RGB color filters are used in combination, an eight-color display can be made. For example, in a display at a reception waiting time for a portable phone, often, even such a degree of an eight-color display is sufficient, and no power is consumed, thus presenting the advantage in that the battery lifetime can be increased.
However, even for portable devices, there are cases in which higher quality moving images and full-color display are desired. Recent portable phones have functions for browsing Web content and for transmitting/receiving images. In addition, in next-generation portable phone services, transmission and reception of moving images will become possible. Because of such increased functionality, it has become necessary for monitor displays to have higher image quality and to be capable of a full-color display. A two-gradation display is insufficient for such applications, and naturally, full-color multi-gradation displays are necessary for future displays in portable phones and portable information terminals.